Embodiments of the invention relate generally to magnetic resonance (MR) imaging, and more specifically, to an RF shield of an RF coil assembly configured to accommodate positioning of elements therein, such as a positron emission tomography (PET) detector array for use in a hybrid PET-MRI system.
PET imaging involves the creation of tomographic images of positron emitting radionuclides in a subject of interest. A radionuclide-labeled agent is administered to a subject positioned within a detector ring. As the radionuclides decay, positively charged particles known as “positrons” are emitted therefrom. As these positrons travel through the tissues of the subject, they lose kinetic energy and ultimately collide with an electron, resulting in mutual annihilation. The positron annihilation results in a pair of oppositely-directed gamma rays being emitted at approximately 511 keV.
It is these gamma rays that are detected by the scintillators of the detector ring. When struck by a gamma ray, each scintillator illuminates, activating a photovoltaic component, such as a photodiode. The signals from the photovoltaics are processed as incidences of gamma rays. When two gamma rays strike oppositely positioned scintillators at approximately the same time, a coincidence is registered. Data sorting units process the coincidences to determine which are true coincidence events and sort out data representing deadtimes and single gamma ray detections. The coincidence events are binned and integrated to form frames of PET data which may be reconstructed into images depicting the distribution of the radionuclide-labeled agent and/or metabolites thereof in the subject.
MR imaging involves the use of magnetic fields and excitation pulses to detect the free induction decay of nuclei having net spins. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a RF magnetic field (excitation field B1) which is in the x-y plane, i.e. perpendicular to the DC magnetic field (B0) direction, and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
In combination PET-MRI systems, the PET detector array needs to be placed outside the RF shield associated with the MRI scanner, in order to shield the sensitive detector array from the high amplitude RF fields of the MRI scanner. As set forth in U.S. patent application Ser. No. 13/461,985, filed May 2, 2012 by General Electric Company, in a combined PET-MRI scanner, the RF shield and RF coil former may be constructed with an indented portion to allow space for a PET detector array to be positioned therein. The RF shield and RF coil former have a stepped configuration to allow for the indented portion to be formed in the center of the RF shield.
As further set forth in U.S. patent application Ser. No. 13/461,985, a typical RF shield may be made from a stainless steel mesh material configured to conform to the stepped configuration of the RF coil former. The steps of the RF coil former are cylindrical but have different diameters. Thus, the indented portion usually has the smallest diameter, while the portions of the RF coil former farthest from the indented portion usually have the largest diameter. A series of steps leading from the largest diameter to the smallest diameter may be formed in the RF coil former to accommodate the PET detector as desired.
The RF shield, when constructed of stainless steel mesh, is typically formed with multiples pieces of overlapping mesh such that separate pieces can conform to the steps of different diameters. To join the multiple pieces together to form a single RF shield, the overlapping edges of the separate pieces are cut to allow overlapping portions to extend toward the adjacent step. The overlapping portions extending toward each other from the different steps are soldered together to allow RF conductivity between the adjacent steps. In this manner, the RF shield for the RF coil former may be constructed to conform to the various steps in the RF coil former. Constructing the RF shield in this manner, however, is typically very costly due to the labor intensive work needed to form the RF shield.
It would therefore be desirable to provide an RF shield and manufacturing process for a stepped RF coil former of a combined PET-MRI scanner that overcome the aforementioned drawbacks.